Electron discharge apparatus



Oct. 8, 194 L. c. PETERSON ELECTRON DISCHARGE APPARATUS 2 Sheets-Shet 1Filed March 31, 1939 FIG.

M W w W L. c. PETERSON BY ATTORNEY Oct. 8. 1940. L. c. PETERSON ELECTRONDISCHARGE APPARATUS 2- Sheets-Sheet 2 Filed March 31; 1939 FIG. 5

FIG. 7

4 B SPACE CHARGE GRID VOLTAGE -8 -4 -2 O CONTROL GRID VOLTAGE FIG. .9

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v CONTROL GRID yam: as

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AT TORNEV Patented Oct. 8, 1940 ELEcTRoN DISCHARGE APPARATUS Liss 0.Peterson, Madison, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationMarch 31, 1939, Serial No. 265,128

6 Claims.

This invention relates to electron discharge apparatus and moreparticularly to multigrid electron discharge devices especially suitablefor the generation and amplification of high frequency waves, forexample up to 50 megacycles.

One general object of this invention is to decrease the input impedanceof electron discharge devices. More specific objects of this inventionare: l To obtain a negative capacitance electronically;

To reduce the active grid loss in electron discharge devices capable ofoperation at high frequencies; and

To increase the frequency range of stable operation for high frequencyelectron discharge apparatus.

In one illustrative embodiment of this invention, electron dischargeapparatus comprises an electron discharge device including a cathode, ananode, a control electrode or grid between the cathode and anode, andone or more space charge or accelerating electrodes or grids between thecathode and the control electrode or grid. The

x space charge or accelerating electrodes or grids are by-passed toground for alternating currents and are maintained at a positive directcurrent potential with respect to the cathode. The control grid ismaintained at a negative potential with respect to the cathode.

In accordance with a broad feature of this invention, the mechanical andelectrical parameters of the electrodes are correlated so that anegative input capacitance for the electron discharge device obtains.More specifically, in accordance with a feature of this invention, thecontrol electrode and the space charge or accelerating electrode nearestthereto are so spaced relative to one another and to the cathode and aremaintained at such direct current potentials that the dielectricconstant of the space charge is negative whereby the capacitance betweenthe cathode and the control electrode is negative in sign.

The invention and the foregoing and other features thereof will beunderstood more clearly and fully from the following detaileddescription with reference to the, accompanying drawings in which:

Fig. l is a perspective view'of an electron discharge deviceillustrative of one embodiment of this invention, a portion of theenclosing vessel and of the electrode structure being broken away toshow details of the structure more clearly;

Fig. 2 is a view in section along line 22 of Fig. 1 of the electrodeassembly, showing the form and disposition of the electrodes;

' Fig. 3 is an enlarged fragmentaryview in section along line 3-3 ofFig. 2, of the electrode assembly;

Fig. 4 is a circuit diagram illustrating one manner of operating theelectron discharge device illustrated in Fig. 1;

Fig. 5 is a diagram illustrating the potential distribution between twoelectrodes of fixed potentials for various values of injected current;

Fig. 6 is a diagram-illustrating the relationship between the injectedcurrent and the ratio 1 of the potential minimum to potential of oneelectrode for a given ratio of the potentials between two electrodes;

Fig. 7 is a, graph showing the variation of input capacitance withcontrol electrode or grid potential in the device shown in Fig. 1, forparticular values of the potential of the accelerating or space chargeelectrodes;

Fig. 8 is a graph showing the relationship between the input capacitanceand the potential of the accelerating or space charge electrodes foraparticular value of control electrode bias in the device shown in Fig.1;

Fig. 9 is another graph including a family of curves illustrating theinput capacitance as a function of the control electrode bias for anumber of values of anode potential in the device. shown in Fig. 1;

Fig. 10 is still another graph' showing the input capacitance and theanode current as a function of the control electrode bias in a device ofthe construction shown in Fig. 1; and

Fig. 11 is a view in cross-section, similar to Fig. 2, illustratinganother embodiment of this invention.

Referring now to the drawings, the electron discharge device illustratedin Figs. 1, 2 and 3 comprises an evacuated enclosing vessel I5 having aninwardly extending stem Hi'which supports an electrode assembly. Theelectrode assembly includes a cylindrical metallic anode I! supported bya pair of metallic rods or upr the grids and are suitably locked inplace.

rights l8 extending from and secured to flanges I9 on a split metallicband or collar 20 clamped about the stem It. Electrical connection tothe anode ll may be established through a leading-in conductor 2i sealedin the inner end of the stem I6 and aflixed to one of the rods oruprights l8.

Mounted within the anode and concentric therewith are a cathode, a pairof accelerating or space charge grids 22 and 23 and a control grid 24.The cathode may be of the indirectly heated equipotential type andinclude a heater filament 25 encased in insulating material 26, and ametallic sleeve 21, the outer surface of which isv coated with anelectron emissive material. It will be understood, of course, that othertypes of cathodes, for example filamentary,

may be employed. Each of the grids 22, 23 and 24 comprises a wire helixcarried by a'pair of the metallic rods or uprights 28, the uprightslying in a diametral plane of the anode as shown clearly in Fig. 2. Thegrids may be of other forms, for example reticulated or slottedcylinders. As illustrated in Fig. 3, corresponding turns of the severalgrids may be in alignment. Preferably, the openings in the grids,particularly those in the control grid, are very small to minimize theeflect of the anode upon the cathodecontrol grid region. In anillustrative structure,

such as shownin Fig. 1, the grid laterals may be of 7-mil wire andspaced .032 inch center to center, the electrode spacings being'as shownin Fig. 3.

fixed, as by welding, to the anode, and the upper insulating member maybe maintained in position by bent-over tabs 33 integral with the anode.

Preferably, only the portion of the cathode sleeve 27 between theinsulating members 3| is coated with electron emissive material so thatall electrons emanating from the cathode are directed toward the grids.

During operation of the device, 'theanode l1 is maintained stronglypositive with respect to the cathode 27, as by a battery 34, and thecontrol grid 24 is biased negatively, as-by a battery 35.

, 'Ihe accelerating or space charge grids 22 and 23 may be connectedtogether and to an intermediate point on the battery 34 and thusmaintained positive with respect to the cathode'21, and-.may beIcy-passed toground for alternating potentials by a large capacitycondenser 36. An input signal may be applied to the control grid 24through a suitable transformer 31 and the-outputcircuit may include vacondenser, 38. If the device is utilized as an amplifier with a;resistance or inductance load, preferably, as illustrated in Fig. 11, anadditional .or shield grid 40 "is provided between the,control grid andthe'anode' to prevent reaction of changes in the anode potential uponthe input impedance. I

It has been determined that, for an electron discharge device, forexample of the construction shown in Fig. 1 and'desc'ribed heretofore,space" charge conditions may be established whereby a negative inputcapacitance obtains. The factors determining the requisite conditionsfor the existence of such negative capacitance may be seen from thefollowing considerations.

It is known, of course, that a variety of space charge conditions mayexist between two electrodes. For example, as illustrated in Fig. 5, inthe region between two parallel planes A and B having potentials E1 andE2, respectively, E1 being greater than E2, in a vacuum and if noelectrons are present, the potential distribution may be represented bya straight line I. If an electron current is injected into this region,from A toward B, and normal to the planes A'and B, the lineardistribution no longer obtains and the potential distribution may be asindicated by the curve 2. If the injected current is increased, apotential minimum develops, as indicated by E0 on curve 3. Astillfurther increase to a certain critical value in the injected currentresults in a decrease'in the potential minimum, as indicated by thecurve 4. When this state has been-established, any further increase,even though slight, in the injected current, causes the potentialminimum to fall to zero, as indicated by curve and a virtual cathode isformed. As the current is increased further, the virtual cathode movesto-, Ward the electrode. A. e

As illustrated in Fig. 6, for a fixed value of. the

ratioof r q E less than unity, a definite relationship exists betweenthe injected current, plotted as 'absci ssae,

and the ratio of the potential minimum, E to I the potential, E1, at theplane of injection, plotted as ordinates. In this figure, the areaembraced by the dotted line X corresponds to a region in which nopotential minimum occurs so that within this region the potentialdistribution departs very slightly, if at'all, from that in a free spaceregion. V

In the full line curve in Fig. 6, the point a corresponds to a value ofinjected current at which a potential minimum has just-set in.- As theinjected current is increased, the curve slopes downwardly to a point 11which corresponds to the critical value of the injected current. Anyfurther increase in the current, as pointed out here: tofore, causes thepotential minimum to fall to zero. It will be seen from Fig. 6 that thevalues.

of injected current between those corresponding to c and d, and b, twopotential distributions are possible, namely that corresponding to theupper part db of the curve and that corresponding to It can be shownthat the the lower part be. capacitance is positive for the upper partof the curve and negative for thelower part. I

If now a negative electroda'such as a nega-. tively biased control grid,is interposed between the positive electrodes A and-.B,and theelectrodes A and B are b-y-passed to ground so that their alternatingcurrent potentials are zero, it: can be shown that the capacitancebetween this negative electrode and the positive electrode A willbenegative for space charge conditions in a regioncorresponding to a partof the full line curve in Fig.6 between a .and b. The'location of thepoint to the left of b will be determined bythe direct electrondischarge device having a negatively" biased control grid between apositive sp-ace charge grid and the-anode may be expressed by therelation I v where c=the capacitance,

C =the capacitance between the control grid and a plane immediatelyadjacent thereto whereat the electron stream is substantially uniform,

Co=iis the cold capacitance, 6 being the permittivity of a vacuum=8.8510'- farads/cm., and d .the distance between the control grid and theaccelerating or space charge grid nearest thereto,

per square centimeter, Zc=lO and m the electron mass in grams,

t1=the electron transit time between the control grid and the spacecharge grid nearest thereto, and

vz=the direct current electron velocity in the immediatevicinity of thecontrol grid.

The capacitance Cg, it is apparent, is inherently large so that thefirst term on the right-hand side of Equation 1 is relatively small. 7

It will be seen from Equation 1 that the capacitance 0 will be negativeif the second term on the right-hand side of the equation is negativeand numerically greater than the first term.

The capacitance given by the second term changes from positive tonegative, 1. e., passes through zero, when It being the injected currentin amperes fin J0 2V2" or, in other words, when The electron velocity112 is determined by the relation Jut where oi The direct currentelectron velocity at the space charge grid nearest the control grid, and

ai the electron acceleration at this space charge grid.

Hence, in order that the capacitance may be negative, it is necessarythat Both in and ii are inherently positive so that in order that therelation given by (5) may obtain, the acceleration a1 must be negative.

Therefore, it will be seen that a negative dielectric constantnecessitates a finite electron velocity and a retarding field at thesurface at which the electron current is injected. In other words, as

to the field, in order that the input capacitance may be negative, it isnecessary that the potentials upon the anode, control grid and theaccelerating or space charge grid be such that the effective field uponthe electrons at this space V1=the direct current potential in the planeof the space charge grid nearest the control ri V2=the direct currentpotential in the plane of the control grid, and Vo=the potentialminimum.

If, the dielectric constant is plotted in terms of these functions, itwill be found that theoretically a negative dielectric constant isobtainable in perfectly stable space charge regions and that as theparameter Ris made smaller the dielectric constant passes through zeroand becomes negative for smaller values of Q. It will be found also thatin the regions of negative dielectric constant, the constant issubstantially independent of the space charge parameter Q. Experienceand tests have indicated, however, that in order to obtain a negativecapacitance it is necessary that the conditions be such that a virtualcathode might form in the region between the space charge and controlgrids. It appears that the regions of negative capacitance set in eitherwhen a virtual cathode begins to form or when such virtual cathodebegins to break up. It has been established that these requisiteconditions for negative capacitance will obtain if the distance betweenthe control grid and the space charge grid is 1.5 times or more thedistance between the cathode and this space charge grid and thepotentials of the control and space charge grids are properlycorrelated.

In an electron discharge device of the construction shown in Fig. 1wherein the electrodes were spaced as indicated in Fig. 3 and the spacecharge grids 22 and 23 were tied together and operated at the samedirect current potential, the input capacitance was found to vary asillustrated in Figs. 7, 8 and 9. For the curves shown in Figs. 7 and 8,the anode H was maintained at 160 volts positive with respect to thecathode 21 and an alternating voltage of 0.18' volt root mean square of50 kilocycles was applied to the control grid 24. With the space chargegrids 22 and 23 maintained at a direct current potential of 18 voltspositive with respect to the cathode, as shown in Fig. '7, the inputcapacitance is negative for negative control grid bias between about 2.5and 1.5 volts. For a negative bias of 2 volts, the capacitance increaseswith both positive and negative increments in control grid potential. Asshown in Fig. 8, with the control grid biased at 2 volts negative, theinput capacitance is negative for potentials upon the space charge grids22 and 23 (tied together) between approximately 16.5 and 18.5 voltspositive.

As indicated in Fig. 9, the negative input capacitance may be obtainedfor various values of anode potential. The curves shown are for acontrol grid bias of 2 volts negative and for a all.

positive v potential of 18 volts upon the space charge grids. In thisfigure the legent Ep indicates anode potential.

It has been found also that the negative input capacitance may beaccompanied by a negative input resistance and a negativetransconductance. A typical transconductance curve for a device of theconstruction shown in Fig. 1 is illustrated in Fig. from which it may beseen that the transconductance becomes negative in the region justbeyond the region of negative capacitance. The

resistance becomes negative at approximately the same point at which thenegative input capacitance appears.

The attainment of such a negative resistance, it will be apparent, is ofconsiderable advantage in amplifier and oscillator devices, particularlythose operable at high frequencies, in that it enables compensation foractive grid loss.

The negative input capacitance may be utilized either per se as animpedance or in a variety of applications. For example, in amplifiers,such capacitance may be utilized to compensate for parasiticcapacitances and thereby extend the frequency range of stable operation.

Although a specific embodiment of the inventionv has been shown and,described, it will be understood that various modifications may be madetherein without departing from the scope and spirit of this invention asdefined in the appended claims.

What is claimed is:

1. Electron discharge apparatus comprising a cathode, an anode, a spacecharge electrode between said cathode and said anode, a controlelectrode between said space charge electrode and said anode, meansmaintaining said anode at a positive potential with respect to saidcathode, and means maintaining said space charge electrode at a positivepotential and said control electrode at a negative potential withrespect to said cathode, said space charge and control electrodes beingso spaced relative to one another and to said cathode and the potentialsappliedv thereto by said second means being such that a potentialminimum obtains in the region between said control and space chargeelectrodes and the effective field at said space charge electrodeisretarding with respect to electrons flowing through said space chargeelectrode toward said control electrode, whereby the capacitance betweensaid cathode and said control electrode is negative in sign.

2. Electron discharge apparatus in accordance with claim 1 wherein thepotentials of said space charge and control electrodes are such thatsaid capacitance increases with both positive and negative increments inthe potential of said control electrode.

3. Electron discharge. apparatus comprising a cathode, an anode, a spacecharge electrode between said cathode and said anode, a controlelectrode between said space charge electrode and 4. Electron dischargeapparatus comprising a cathode, an anode, a space charge electrodebetween said cathode and said anode, a control electrode between saidspace charge electrode and said anode, means maintaining said spacecharge electrode at a positive potential with respect to said cathode,and means maintaining said control electrode at a negative potentialwith respect to said cathode, said positive and negative potentialsbeing such that an unstable space charge obtains in the region betweensaid control and space charge electrodes and the efiective field at saidspace charge electrode is retarding with respect to electrons flowingthrough said space charge electrode toward said control electrode.

5; A high frequency amplifier comprising a cathode, an anode, a spacecharge electrode between said cathode and said anode, a controlelectrode between said space charge electrode and said anode, an inputcircuit connected to saidcathode and said control electrode includingmeans for biasing said control electrode negatively with respect to saidcathode, an output circuit connected to said cathode and said anode,means maintaining said space charge electrode at a positive directcurrent potential with respect to said cathode, and means for by-passingsaid space charge electrode to said cathode for alternating currents,the spacing between said space charge and control electrodes and saidcathode and the bias upon said control electrode and the direct currentpotential of said space charge electrode being such that the dielectricconstant of the region between said space charge and control electrodesis negativein sign.

6. Electron discharge apparatus comprising a cathode, a cylindricalanode encompassing said cathode and coaxial therewith, cylindrical spacecharge and control grids between said cathode and said anode and coaxialtherewith, each of said grids including a plurality of closely spacedwires, corresponding wires of said grids being in alignment, saidcontrol grid being between said space charge grid and said anode andspaced from said space charge grid a distance at least 1.5 times asgreat as the distance between said cathode and said space charge grid,means applying a positive potential to said space charge grid, and meansapplying a negative bias to said control grid, said potential and biasbeing such that an unstable space charge obtains in the region betweensaid grids.

' LISS C. PETERSON.

